It is known that, in internal combusion engines with fixed timing of the distribution, the selected angles of advance of the opening and lag of the closing of the valves, particularly the inlet valves, are optimal only at certain speeds, while at other speeds they represent a compromise which is sometimes barely acceptable.
Thus it is found that the inlet valves are sometimes opened while the discharge valves are still open (crossover). While this promotes the discharge of the spent gases from the cylinders at certain speeds, at other speeds it causes a return of the spent gases from the exhaust system to the cylinders and even into the intake pipes, thus decreasing the volumetric efficiency of the engine.
In some cases, moreover, in order to improve the refilling of the cylinders at high speeds, the inlet valves are closed by inertial and resonance effects after the start of the compression storke.
However, at low speeds, owing to the reduced inertia of the charge entering the cylinders, at the start of the compression stroke a considerable part of the charge may flow back into the intake pipes through the inlet valves which are still open, with partial evacuation of the cylinders.
To eliminate these disadvantages, it has been proposed that the intake pipes should be provided with non-return valves capable of permitting the flow of air or mixture into the cylinders and of preventing a reverse flow to the exterior, restraining the charge from flowing back from the cylinder into the intake pipes.
For this purpose, numerous embodiments of non-return valves of the flap type known in the previous art have been used. However, these have the disadvantage of feeding into the intake pipes volumes of air which flow in parallel entry directions as a result of the uniform orientation of the exit direction of the air flow from the flap units of the valves. The volumes of air supplied in this way have a very wide flow section, a low relative density, and a low velocity of entry into the combustion chambers of the cylinders, particularly at low speeds, and consequently the air flow may become turbulent, thus counteracting the desired injection of mixture into the combustion chambers of the cylinders, while promoting, as a result of the low inertia of the air supplied in this way, the return of air into the feed pipe during the time interval required for the cylinders to enter the compression phase and close the inlet valves.